Phosphorus trichloride is usually prepared by the direct reaction of phosphorus and chlorine in a reactor containing phosphorus trichloride. The phosphorus trichloride produced is generally removed from the reactor by distillation through utilization of the heat of reaction. The reaction takes place in phosphorus trichloride to which elemental phosphorus is added. The phosphorus dissolves into the phosphorus trichloride. Chlorine is sparged into the phosphorus trichloride to react with dissolved phosphorus. Sparging aids in dissolving the phosphorus into the phosphorus trichloride.
There are several possible reactions which can take place in the reactor which are described by the following reactions: EQU P.sub.4 +6Cl.sub.2 .fwdarw.4 PCl.sub.3 EQU Cl.sub.2 +PCl.sub.3 .fwdarw.PCl.sub.5 EQU P.sub.4 +6PCl.sub.5 .fwdarw.10 PCl.sub.3
From the above illustrated reactions, it is clear that the undesired production of phosphorus pentachloride may occur. To reduce the amount of production of this undesired product, the reaction is usually controlled by the amount of chlorine introduced into the reactor allowing for an excess of phosphorus with respect to chlorine to be present in the phosphorus trichloride.
Control of the reaction is important because phosphorus trichloride is a hazardous chemical which can severely burn skin, eyes, and mucous membranes. Vapors from minor inhalation can cause severe respiratory ailments depending upon the degree of exposure. Also, phosphorus trichloride is highly toxic and it reacts violently with water generating dangerous acids and phosphines. The heat of the reaction for the formation of phosphorus pentachloride from phosphorus trichloride and chlorine is less that the formation of phosphorus trichloride from phosphorus and chlorine. This drop in heat generation can be used to detect deficiency of phosphorus in the reactor but this mechanism is indirect. In the usual case, the reaction conditions are controlled by the rate of chlorine addition into the reactor. In general, such control is effective to prevent formation of undesirable amounts of phosphorus pentachloride. The amount of excess phosphorus in the phosphorus trichloride is maintained at a relatively high level to minimize the formation of phosphorus pentachloride and thus provide for safe and efficient operation of the process. Safety considerations are a major factor when dealing with the chemicals involved.
The problem of controlling the reaction of phosphorus and chlorine has produced various analytical methods or schemes based upon the periodic determination of elemental phosphorus in the reaction zone of the reactor. One method of determination of phosphorus is disclosed in Chem. Tech (Leipzig) Vol. 24, No. 6, p. 363 (1972) by Rudolph Schumann. In this method a measured sample of the phosphorus trichloride solution is withdrawn from the reactor and poured into a Dewar flask containing bromine dissolved in carbon tetrachloride. The temperature of the solution before addition of the sample is noted, and after thorough mixing, the temperature is again noted. The temperature difference is a rough measure of the phosphorus concentration as determined by comparison with a calibration curve.
Another method of analysis for phosphorus and phosphorus chlorides is gas-chromatography. Such a method is reported by M. M. Kirievskaya et al. in a publication entitled "Gas-chromatographic Determination of White Phosphorus and of Certain Phosphorus Chlorides" appearing in Zhurnal Prikladnoi Khimii, Vol. 45, No. 9, pp.2074-2076, (1972). A chromatograph was employed to determine white phosphorus and various phosphorus chlorides with a 5000.times.3 mm column packed with Celite 545 and SKT-FT-100.times. methylfluoropropylsilicone rubber. The column temperature was 150.degree. C. and a thermoconductivity detector was used. The method was based upon mixtures of reagent grade materials mixed together in the laboratory.
The above examples of analyses require the transport of samples to the analytical apparatus and considerable delay in obtaining results. There is needed a rapid, safe and yet accurate method for controlling large scale commercial processes for the production of phosphorus trichloride.
It is therefore important that the reactor in which phosphorus trichloride is produced is very secure and is operated with control so as to assure both maximum safety as well as efficiency.
The sampling of the reaction mixture in which phosphorus trichloride is produced is a particularly difficult task when desiring to provide a representative sample to the analytical system for monitoring the reaction on a continuous basis. As noted above the reactor contains materials which must be handled with great care and contained within a closed vessel. One way to prevent leaks and spills from the reactor is to withdraw the sample from the top of the reactor. The reactor contains a boiling liquid making it difficult to withdraw a sample from the top without flashing of the liquid. Flashing occurs due to reduced pressure resulting in vapor being admitted into the sample system rather than a representative sample of the liquid reaction mixture.